opening a file f:

fd = open(f,RW) ->
  adr = mmap (f,RW)
  filelist[fd].mem = adr


reading from a file f:

read (fd, buf, count) ->
  adr = filelist[fd].mem
  offset = filelist[fd].pos
  filesize = filelist[fd].size
  if (offset+count > size) count = count - ...
  memcpy (buf, adr+offset, count)
  offset += count
  return count

lseek (fd, offset, OPT) ->
  calculate absolute offset from current filelist[fd].pos
  filelist[fd].pos = abs_offset


writing to a file f:

- problem with appending to a file
- need to have a more flexible mmap()
- or use a maximal file size and create a
  sufficiently large mapping
- otherwise like read()